1. Field of the Invention
This invention relates to an angle measuring device, and in particular, to an angle measuring device for measuring the turning angle of a crankshaft in an internal combustion engine in order to control the ignition timing, etc. thereof.
2. Description of the Related Art
In order to control the ignition timing, etc. of an internal combustion engine, an angle measuring device is used which is adapted to emit a predetermined pulse in synchronization with rotations of the crankshaft of the engine. FIG. 1 shows an example of conventional devices of this type. The device shown comprises a shaft 1 which rotates in synchronization with the rotation of the crankshaft (not shown), a rotating disc 2 attached to the shaft 1, light-emitting elements 3 and 4, light-receiving elements 5 and 6, and an electronic circuit 7 including a semiconductor signal-processing circuit.
FIG. 2 is a plan view of the rotating disc 2 of FIG. 1. As shown in the drawing, a plurality of slits 8a and 8b are provided at predetermined intervals along the circumference of the rotating disc 2, respectively.
FIG. 3 is a circuit diagram schematically showing the signal processing section of the electronic circuit 7 of FIG. 1. The signal processing section shown includes an input terminal 9, a resistor 10 connected in series to the input terminal 9, a capacitor 11 one end of which is connected to the resistor 10 and the other end of which is grounded. The reference numeral 12 indicates a signal processing circuit connected to the one end of the capacitor 11 and comprising a transistor 12a, etc.
In this conventional angle measuring device, which has the above-described construction, the rotating disc 2 shown in FIG. 2 rotates by being driven by the shaft 1, in synchronization with the rotation of the associated crankshaft. Light is input to the light-receiving elements 5, 6 and causes them to emit signals only when slits 8a and 8b are positioned between them and the light-emitting elements 3, 4. These signals are applied to the input terminal 9 shown in FIG. 3 so as to be processed. However, when, in the signal processing section shown in FIG. 3, high-frequency surge noise due to ignition are superimposed on signals to be applied to the input terminal 9, an electric current flows to the ground through the resistor 10 and a diode between the base and emitter of the transistor 12a in the signal processing circuit 12. Due to the presence of the diode , an inverse electric current flow does not occur. As a result, the high-frequency noise is rectified and smoothed into pulses having a large width by the diode in the transistor 12a, the resistor 10 and the capacitor 11. That is, the impedance between the input terminal of the signal processing circuit 12 and the power source and the impedance between the input terminal and the ground are ill balanced. Therefore, a great noise which is polarized in either the positive or negative direction is generated, resulting in the signal processing circuit 12 being subject to malfunctions.